The recent recognition that the world is rapidly depleting its fossil fuel reserves has inspired much development activity directed toward trapping the energy of the sun and converting it into a form that will help reduce reliance upon fossil fuels as a primary energy source. Many such proposed devices have incorporated glass tubes and particularily, double-walled energy absorbing glass tubes are disclosed in a number of prior art patents wherein two glass tubes are concentrically disposed but radially spaced relative to each other and the axial end portions thereof sealed so that the annular space between the inner and outer glass tubes may be evacuated. An energy absorbing coating is then applied to the external surface of the innermost glass tube and this coating serves to concentrate most of the energy that is incident upon the outer glass tube within the inner glass tube, which energy may be converted to utilitarian purposes by passing a suitable heat transfer fluid through the bore of the inner glass tube.
Typical of these devices are U.S. Pat. Nos.: 980,505 to Emmet, 1,946,184 to Abbot and 2,141,330 to Abbot. While all of these early patents recognized the advantages of a double-walled glass tube having an evacuated annulus as an efficient heat absorber, there was also recognition of the fact that the inherent heating of the inner tube element of the double-walled tube construction resulted in unequal thermal expansion of the inner glass tube relative to the outer glass tube. Hence it was proposed that bellows-type joints or sections be incorporated in the inner glass tube to provide for a means for automatically absorbing the unequal expansion. However, all of the prior art attempts to incorporate an integral bellows configuration in the inner glass tube resulted in a reduction in strength of the inner tube as well as a reduction in the effective diameter of the inner glass tube which, being the primary heat absorbing element, accordingly effected a reduction in the solar heat absorbing efficiency of the resulting double-walled tube.